The present invention relates to display devices. In particular, the present invention relates to liquid crystal displays having a input polarizer and an output polarizer, and which have an extended high temperature operating range.
A liquid crystal display typically includes a light source, an input polarizer, a liquid crystal module, and an output polarizer. Light from the light source travels to the input polarizer, which polarizes the light. The polarized light then travels from the input polarizer to the liquid crystal module.
A liquid crystal module includes liquid crystal compositions contained between two substrates. As polarized light passes through the liquid crystal composition, the liquid crystal molecules gradually change the light's plane of vibration to match the angle of the liquid crystal. When the light reaches the exit side of the liquid crystal module, it will vibrate at the same angle as the molecules of the liquid crystal.
Liquid crystal modules operate through the use of matrix arrays of circuits. These matrix arrays allow control over the angle of the liquid crystal molecules at various locations along the array. There are many different variations of matrix arrays. A very efficient and widely used form is found in active matrix displays. Active matrix displays incorporate matrix arrays containing thin film transistors located at each array intersection. Thin film transistors are switching elements that allow distinct control over individual pixels in the matrix. When a particular pixel is to be manipulated, the row where the pixel is located is turned on and a voltage is applied to the column where it is located. Because all of the other rows were turned off, only the given row that is turned on receives the voltage. When the voltage is received by a certain pixel, the liquid crystal molecules change their twist angles in proportion to the voltage and modify the plane of vibration of the incoming light. The light then travels to the output polarizer, whose polarization direction is oriented at 90° to the input polarizer. Based on the angle of rotation of the light polarization while passing though the liquid crystal module, a percentage of the polarized light will pass through the output polarizer.
Because of the need for optimal polarization to obtain an effective liquid crystal display, high efficiency of the input and output polarizers is required. Polarizers absorb part of the light transmitted through the liquid crystal display. A common form of polarizer is a high efficiency iodine-type polarizer, which is well known for its excellent optical performance. High efficiency iodine-type polarizers incorporate long molecular chains of iodine molecules, which are the mechanism for the light absorption. In addition to the iodine molecular chains, iodine-type polarizers are typically composed of several polymeric layers that include materials such as polyvinyl alcohol (PVA) and cellulose triacetate (CTA). The CTA is used on both sides of the PVA layer to protect the iodine molecules in the PVA layer against degradation from exposure to moisture. For special applications, additional protection can be provided against more demanding environments. Such additional protection may include laminating glass sheets over the CTA layers, in order to further encapsulate the polarizer and protect it against environmentally caused degradation. Adhesive layers may also be incorporated to bind multiple layers together.
A common structure of iodine-type polarizer is a film polarizer. A film polarizer includes a stretched layer of a polymeric substrate, such as polyvinyl alcohol. The polymeric substrate such as PVA is stretched along an axis, so that the long molecular iodine chains will align themselves on the surface of the PVA in a direction parallel to the stretched axis. The iodine molecules are absorbed onto the stretched polymeric substrate in the form of long molecular chains. These long molecular chains of iodine, that are oriented parallel to the stretched axis of the PVA, absorb the incoming light that is polarized parallel to the stretched axis. The remaining light, that is transmitted through the iodine type polarizer, is polarized perpendicular to the direction of the stretched axis.
Despite their excellent optical performance, iodine-type polarizers reach a performance limit at about 90° C. Above 90° C., iodine-type polarizers degrade in optical performance, due to a heat driven reaction. Above 90° C., the heat can initiate the break up of the long molecular iodine chains that provide the means for absorbing polarized light. In this process, the polarizer is bleached because it no longer absorbs polarized light. One known mechanism for breaking the molecular iodine chains is water absorption. It has been hypothesized that the breakdown of the iodine molecular chains is due to the release of the residual water molecules in the polyvinyl alcohol substrate.
Avionics is one of the many fields that utilize liquid crystal displays. Avionics liquid crystal displays have made use of high efficiency iodine polarizers for both input and the output polarizers. In certain avionics applications, liquid crystal displays are directly exposed to sunlight, which heats the front of the display. This creates a thermal gradient from the front of the display to the back of the display. As a result of this thermal gradient, the temperature of the output polarizer that is closest to the front of the display is greater than the input polarizer that is closest to the rear of the liquid crystal display. The effect of this is the output polarizer closest to the front of the display can be exposed to temperatures up to about 110° C., while the input polarizer closest to the rear of the liquid crystal display is slightly cooler (about 85–90° C.). Therefore, the input iodine type polarizer at the rear of the display is capable of high performance for a much greater duration than the output iodine type polarizer at the front of the display.
Other high efficiency polarizers are known which have very good optical performance, but not quite as good as the high efficiency iodine type polarizers. A polarizer is needed that can withstand high temperatures up to 110° C. and maintain its optical performance without degradation. This high temperature type polarizer will need to provide optical performance comparable to the current high performance iodine-type polarizers in terms of transmission and efficiency. An example of a high temperature polarizer is the high efficiency dye-type polarizer. Dye-type polarizers are stable at high temperatures, but have lower transmission compared to iodine type polarizers. For example, currently the best iodine type polarizer has a transmission T1=42.7% (Nitto EG 1224DU, for example), while currently the best dye-type polarizer has a T1=39.7% (Sumitomo ST 1822AP, for example). The dye-type polarizer transmits only 93% of the polarized light compared with the Nitto iodine-type polarizers. Using two dye-type polarizers as the input and output polarizers in place of iodine type polarizers would reduce the transmission of polarized light through the liquid crystal display to about 85% compared with the Nitto iodine-type polarizers. Similarly, by using two dye-type polarizers as the input and output polarizers in the cross configuration (i.e., polarization directions of the polarizers oriented at 90° to one another to maximize the extinction of light), the transmission is reduced to 0.02%. With two crossed iodine polarizers, the transmitted light is reduced to 0.01%, which is twice as good as the two crossed dye-type polarizers.